The innate immune system provides the body with a first line defense against invading pathogens. In an innate immune response, an invading pathogen is recognized by a germline-encoded receptor, the activation of which initiates a signaling cascade that leads to the induction of cytokine expression. Innate immune system receptors have broad specificity, recognizing molecular structures that are highly conserved among different pathogens. One family of these receptors is known as Toll-like receptors (TLRs), due to their homology with receptors that were first identified and named in Drosophila, and are present in cells such as macrophages, dendritic cells, and epithelial cells.
There are at least ten different TLRs in mammals. Ligands and corresponding signaling cascades have been identified for some of these receptors. For example, TLR2 is activated by the lipoprotein of bacteria (e.g., E. coli.), TLR3 is activated by double-stranded RNA, TLR4 is activated by lipopolysaccharide (i.e., LPS or endotoxin) of Gram-negative bacteria (e.g., Salmonella and E. coli 0157:H7), TLR5 is activated by flagellin of motile bacteria (e.g., Listeria), TLR7 recognizes and responds to imiquimod and TLR9 is activated by unmethylated CpG sequences of pathogen DNA. The stimulation of each of these receptors leads to activation of the transcription factor NF-κB, and other signaling molecules that are involved in regulating the expression of cytokine genes, including those encoding tumor necrosis factor-alpha (TNF-a), interleukin-1 (IL-1), and certain chemokines. Agonists of TLR-7 are immunostimulants and induce the production of endogenous interferon-a in vivo.
There are a number of diseases, disorders, and conditions linked to TLRs such that therapies using a TLR agonist are believed promising, including but not limited to melanoma, non-small cell lung carcinoma, hepatocellular carcinoma, basal cell carcinoma, renal cell carcinoma, myeloma, allergic rhinitis, asthma, COPD, ulcerative colitis, hepatic fibrosis, and viral infections such as HBV, Flaviviridae viruses, HCV, HPV, RSV, SARS, HIV, or influenza.
The treatment of Flaviviridae virus infections with TLR agonists is particularly promising. Viruses of the Flaviviridae family comprise at least three distinguishable genera including pestiviruses, Flaviviruses, and hepaciviruses (Calisher, et al., J. Gen. Virol., 1993, 70, 37-43). While pestiviruses cause many economically important animal diseases such as bovine viral diarrhea virus (BVDV), classical swine fever virus (CSFV, hog cholera) and border disease of sheep (BDV), their importance in human disease is less well characterized (Moennig, V., et al., Adv. Vir. Res. 1992, 48, 53-98). Flaviviruses are responsible for important human diseases such as dengue fever and yellow fever while hepaciviruses cause hepatitis C virus infections in humans. Other important viral infections caused by the Flaviviridae family include West Nile virus (WNV) Janpanese encephalitis virus (JEV), tick-borne encephalitis virus, Junjin virus, Murray Valley encephalitis, St Louis enchaplitis, Omsk hemorrhagic fever virus and Zika virus. Combined, infections from the Flaviviridae virus family cause significant mortality, morbidity and economic losses throughout the world. Therefore, there is a need to develop effective treatments for Flaviviridae virus infections.
The hepatitis C virus (HCV) is the leading cause of chronic liver disease worldwide (Boyer, N. et al. J Hepatal. 32:98-112, 2000) so a significant focus of current antiviral research is directed toward the development of improved methods of treatment of chronic HCV infections in humans (Di Besceglie, A. M. and Bacon, B. R., Scientific American, October: 80-85, (1999); Gordon, C. P., et al., J. Med. Chem. 2005, 48, 1-20; Maradpour, D.; et al., Nat. Rev. Micro. 2007, 5(6), 453-463). A number of HCV treatments are reviewed by Bymock et al. in Antiviral Chemistry & Chemotherapy, 11:2; 79-95 (2000). Currently, there are primarily two antiviral compounds, ribavirin, a nucleoside analog, and interferon-alpha (α) (IFN), that are used for the treatment of chronic HCV infections in humans. Ribavirin alone is not effective in reducing viral RNA levels, has significant toxicity, and is known to induce anemia. The combination of IFN and ribavirin has been reported to be effective in the management of chronic hepatitis C (Scott, L. J., of al. Drugs 2002, 62, 507-556) but less than half the patients given this treatment show a persistent benefit.
HCV is recognized by innate virus-sensing mechanisms that induce a rapid IFN response (Dustin, et al., Annu. Rev. Immunol. 2007, 25, 71-99). It is likely that the sources of the IFN are, at least, the infected hepatocytes and particularly the plasmacytoid dendritic cells (pDC) that highly express TLR 7 receptors and secrete high amounts of IFN. Horsmans, et al. (Hepatology, 2005, 42, 724-731), demonstrated that a once daily 7-day treatment with the TLR 7 agonist isatoribine reduces plasma virus concentrations in HCV infected patients. Lee, et al. (Proc. Natl. Acad. Sci. USA, 2006, 103, 1828-1833), demonstrated that TLR 7 stimulation can induce HCV immunity by both an IFN and IFN-independent mechanisms. These workers also revealed that TLR 7 is expressed in normal as well as HCV infected hepatocytes. These combined results support the conclusion that stimulation of TLR 7 receptors, such as through the administration of a TLR 7 agonist, is a viable mechanism for effectively treating natural HCV infections. Given the need for more effective treatments for HCV infections, there is a need to develop safe and therapeutically effective TLR 7 agonists.